A wide variety of implantable medical devices (IMDs) for delivering a therapy or monitoring a physiologic condition which may employ one or more elongated electrical leads and/or sensors have been clinically implanted or proposed for clinical implantation in patients. A wide variety of electrical stimulation therapy delivery IMDs comprising hermetically sealed, implantable pulse generators (IPGs) and associated electrical leads are implanted in patients' bodies for delivering electrical energy to locations of the body. Such IMDs comprise cardiac pacemakers, implantable cardioverter/defibrillators (ICDs) and muscle, nerve, brain, and organ stimulators, etc.
The leads of such IMDs typically comprise a lead body extending between a proximal lead end and a distal lead end and incorporates one or more exposed electrode or sensor element located at or near the distal lead end. One or more elongated electrical conductor extends through the lead body from a lead connector element formed at a proximal lead end for connection with connector elements of the IPG or monitor and a sensor terminal or electrode located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from any other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductor from body tissue and fluids and prevents physical contact of the conductors with such body tissue and fluids.
Such leads may extend from a subcutaneous implantation site of the IPG or monitor module through a wide variety of pathways into or adjacent to various chambers of the heart, deeply into the brain, into a location within the spine, and into or adjacent other body organs, muscles and nerves. The leads must be formed with small diameter, highly flexible, reliable lead bodies that withstand degradation by body fluids and body movements that apply stress and strain to the lead body and the connections made to electrodes or sensor terminals. As lead bodies are made smaller and smaller and the number of lead conductors is increased or maintained, problems with lead insulation and integrity of lead conductors may become more prevalent. During implantation, lead body insulation can be inadvertently breached or coiled lead conductors can be crushed in minute areas and can be overlooked. Later, these defects may be magnified by exposure to body fluids and result in conductor related condition that is intermittent or which slowly or suddenly manifests itself.
For example, modern implantable cardiac pacemakers and ICDs comprise an IPG implanted subcutaneously remote from the heart and a pacing lead or leads extending from the IPG to a pace/sense electrode or electrodes located with respect to a particular heart chamber to deliver the pacing pulses and sense the cardiac P-wave or R-wave. The lead bodies of such cardiac leads are continuously flexed by the beating of the heart, and other stresses are applied to the lead body in part affected by the implantation route taken between the IPG and the heart chamber or cardiac vessel where the electrodes or sensors are located. Movements by the patient can also cause the route traversed by the lead body to be constricted, whereby shear forces are applied to the lead body sheath and electrical conductors. At times, the lead bodies can be slightly damaged during surgical implantation, and the slight damage may progress in the body environment until a lead conductor fractures and/or the insulation is breached. In most such cases, the effects of lead body damage progress from an intermittent manifestation to a more continuous lead related condition state with lead aging. In extreme cases, insulation of one or more of the electrical conductors may be breached, causing the conductors to contact one another or body fluids resulting in a low impedance or short circuit. Or a lead conductor may fracture and exhibit an intermittent or continuous open circuit resulting in an intermittent or continuous high impedance.
Other problems can arise at the proximal lead end where the electrical connection between the IPG or monitor connector elements and the lead connector elements may be intermittently or continuously disrupted, resulting in a high impedance or open circuit. Usually, such connector open circuit problems result from insufficient tightening of the connection at the time of implantation followed by a gradual loosening of the connection until contact becomes intermittent or open.
In addition, the lead distal end may become dislodged from connection or contact with cardiac tissue, resulting in intermittent or continuous loss of contact of a distal pace/sense electrode with the heart tissue. The dislodgement may leave the electrodes floating in the blood of a heart chamber. Or, "lead penetration" may occur during implantation or chronically wherein the distal end of the lead may be advanced too far into the heart tissue or advances partly through the myocardium. Alternatively, "exit block" may occur, wherein a foreign body reaction, e.g. tissue growth over the pace/sense electrode surface or inflammation of the cardiac tissue adjacent the pace/sense electrode surface increases the pacing or sensing threshold to a level that can result in loss of pacing or sensing.
When these lead problems manifest themselves, they can be collectively referred to for simplicity as a "lead related condition" event though the lead itself is intact. Such lead related conditions may also include a connector open circuit condition or lead dislodgement. It is necessary for the clinician to diagnose the nature of the lead related condition from the available data, test routines that are undertaken, and IMD and patient symptoms. Then, it is necessary for the clinician to take corrective action, e.g., to either replace the lead, reposition the electrodes or sensors or tighten the proximal connection. In severe cases, the lead related condition may result in depletion of the battery energy of the IMD, requiring its replacement.
Certain IPGs and monitors have been clinically used or proposed that also rely on lead borne physiologic sensors that monitor physiologic conditions, e.g., blood pressure, temperature, pH, blood gases, etc. Cardiac pacemakers employing such sensors use the processed sensor signals to regulate pacing characteristics, e.g., pacing rate and/or energy. Open circuit or short circuit lead conductor related conditions or connector or dislodgement related conditions can disable such sensors and compromise monitoring and/or pacing operations dependent upon true sensor output signals.
The ability to sense P-waves or R-waves accurately through a lead can be impaired by any of these lead related conditions. Complete lead breakage impedes any sensing functions, lead conductor fractures or intermittent contact can cause electrical noise that interferes with accurate sensing, and loss of contact of the pace/sense electrodes with responsive cardiac tissue can cause true cardiac signals to be distorted or attenuated.
In the context of cardiac pacing, a delivered pacing pulse "captures" the heart if its delivery through an active, cathodal, pace/sense electrode to the adjacent heart tissue causes or "evokes" a myocardial contraction and depolarization wave that is conducted through the myocardium away from that pace/sense electrode site. The increased impedance of the pacing path or the short circuit of lead conductors due to one of the above-described lead related conditions can reduce the effective pacing pulse energy below that sufficient to capture the heart, resulting in loss of capture (LOC). Commonly assigned U.S. Pat. No. 5,861,012 (Stroebel), incorporated herein by reference, describes several approaches to the determination of the pacing threshold energy that achieves capture and the adjustment of the pacing pulse peak voltage and pulse width to deliver adequate pacing energy to assure capture while avoiding delivery of excess energy that is wasteful of battery energy. Periodically, a pacing threshold test is conducted wherein the pacing pulse width and amplitude are reduced to determine a chronaxie value related to the pacing pulse width sufficient to capture the heart and rheobase value related to pulse amplitude sufficient to capture the heart. Such threshold or LOC test data are stored in memory, and the pacing pulse width and/or amplitude are automatically increased from the threshold levels to provide a "safety margin" to assure capture of the heart.
Similar lead related conditions can occur with cardioversion/defibrillation leads that can result in failure to cardiovert or defibrillate the heart at a programmed shock energy level. The failure of the delivered therapy can be dangerous to the patient and/or can necessitate applying further, higher energy, cardioversion/defibrillation shocks which increases discomfort of the patient and is wasteful of battery energy.
The issue of the integrity of pacing leads and ICD leads has been a serious concern over many years. Certain cardiac IPGs have been provided with the capability of storing EGM and event data prompted by the automatic determination of over sensing and undersensing of cardiac events, LOC events, out of range lead impedance measurements, etc., that can be telemetered to the external programmer when the physician interrogates the IPG or monitor and used by the clinician in evaluating lead function.
The lead impedance data and other parameter data, e.g., battery voltage, bipolar to unipolar lead switch events, error counts, LOC event data, etc., that is telemetered to the external programmer is typically compiled and displayed on a monitor and/or printed out for analysis by the clinician. The clinician may undertake real time IPG parameter reprogramming and testing and observe the monitored surface ECG to try to pinpoint a suspected lead related condition that is indicated by the data and/or patient and/or device symptoms.
Certain external programmers that address the analysis of such data and symptoms include those disclosed in the following U.S. Pat. No. 4,825,869 (Sasmor et al.); U.S. Pat. No. 5,660,183 (Chiang et al.); and U.S. Pat. No. 5,891,179 (Er et al.), all incorporated herein by reference. The '869 patent describes processing a variety of uplink telemetered atrial and ventricular EGM data, stored parameter and event data, and the surface ECG in rule-based algorithms for determining various IPG and lead malfunctions. The '183 patent also considers patient symptoms in an interactive probability based expert system that compares data and patient systems to stored diagnostic rules relating symptoms to etiologies to develop a prognosis. The '179 patent discloses a programmer that can be operated to provide a kind of time varying display of lead impedance values in relation to upper and lower impedance limits. The lead impedance values are derived from pacing pulse current and voltage values and are either measured and stored in the IPG memory at an earlier time or comprise current, real time values that are uplink telemetered to the programmer for processing and display.
The diagnosis of lead related data at a later time in such ways is useful, but it is believed preferable to provide a more immediate response to a lead related condition by the IPG or monitor. The retrieved data may be suspect if a lead related condition causes the stored or real time telemetered data to be inaccurate. The physician may mistakenly rely upon such data to maintain or change programmed pacing parameters and modes, particularly if a lead related condition is intermittent and is not diagnosed.
Many proposals have been advanced to determine if a lead related condition has occurred and to modify the IPG operation and/or to provide a warning that is perceptible by the patient or can be telemetered to the external programmer when the physician interrogates the IPG or monitor. In addition, it has been a goal to automatically detect a lead conductor related condition and respond by switching pacing pathways to use available lead conductors that appear to be functioning properly.
Prior art detection of lead related condition and various IPG responses to such detection are set forth in U.S. Pat. No. 4,140,131 (Dutcher et al.); U.S. Pat. No. 4,549,548 (Wittkampf et al.); U.S. Pat. No. 4,606,349 (Livingston et al.); U.S. Pat. No. 4,899,750 (Ekwall); U.S. Pat. No. 5,003,975 (Hafelfinger et al.); U.S. Pat. No. 5,137,021 (Wayne et al.); 5,156,149 (Iludrlik); U.S. Pat. No. 5,184,614 (Collins); 5,201,808 (Steinhaus et al.); U.S. Pat. No. 5,201,865 (Kuehn); U.S. Pat. No. 5,224,475 (Berg et al.); U.S. Pat. No. 5,344,430 (Berg et al.); U.S. Pat. No. 5,350,410 (Kieks et al.); U.S. Pat. No. 5,431,692 (Hansen et al.); U.S. Pat. No. 5,453,468 (Williams et al.); U.S. Pat. No. 5,507,786 (Morgan et al.); U.S. Pat. No. 5,534,018 (Walhstrand et al.); U.S. Pat. No. 5,549,646 (Katz et al.); U.S. Pat. No. 5,722,997 (Nedungadi et al.); U.S. Pat. No. 5,741,311 (McVenes et al.); U.S. Pat. No. 5,755,742 (Schuelke et al.); and U.S. Pat. No. 5,814,088 (Paul et al.). All of these patents are incorporated by reference.
Most of these patents disclose systems for periodically measuring lead impedance and comparing the impedance measurements with upper and lower impedance values or ranges and either storing the data for later retrieval, and/or changing a pacing or cardioversion/defibrillation path, and/or adjusting the delivered pacing energy, and/or alerting the patient by generating sound or stimulation warning signals. Most of the above-incorporated patents depend on the generation of an impedance reading during a period of time when the pacemaker is not providing a stimulation pulse to the heart or, alternatively, sample and hold some portion or portions of a pacing signal, digitize some characteristic or characteristics inherent in that signal, and have that digitized signal processed by an on-board algorithm or circuit in order to produce an impedance value for the conductor under test. The impedance value is typically compared to upper and lower impedance thresholds, and employed as described above. In most cases, event data comprising the signal value and time and date are stored in memory whenever the impedance value exceeds or falls below the upper and lower impedance thresholds (i.e., the lead impedance is out of range). Certain of the above-incorporated patents, e.g. the '786 patent, also provide monitoring and storage of other parameters of IPG operation, e.g., battery voltage, for later retrieval and analysis by a clinician in an uplink telemetry session. Others of the above-incorporated patents disclose some processing of the lead impedance values within the IPG, and storage of the processed data for later retrieval and analysis by the clinician. The above-incorporated '975 patent discloses measuring unipolar and bipolar lead impedances, incrementing an error counter at least when the bipolar lead impedance value is out of range, and switching to a unipolar lead configuration, if one is available that exhibits a lead impedance value that is in the acceptable impedance range. The above-incorporated '750 patent discloses measuring output energy delivered during a pacing pulse, deriving a lead impedance value therefrom that is compared to a moving average impedance value, and incrementing a first error counter if a series, e.g., three, of such lead impedance values are out of range. In addition, characteristics of sensed heart signals are monitored, and the count of a second error counter is incremented if a series of the sensed heart signals exhibit an abnormality, e.g. an abnormal slew rate that could be due to a lead related condition. The counts are interrogated and displayed by an external programmer in an uplink telemetry session to alert the clinician of a possible lead related condition that should be investigated.
The above-incorporated '742 patent discloses an ICD lead impedance measurement system that measures impedance of all of the cardioversion/defibrillation leads and pacing leads using three leads at a time. A force lead and a measure lead are selected to drive current through a lead under test and to measure the voltage induced in the lead under test. Lead impedance values are derived and compared to upper and lower impedance thresholds. Out of range lead impedance value data causes an invalid flag to be set, may cause a patient warning to be emitted, and is stored as event data for later interrogation and uplink telemetry to the external programmer. The uplink-telemetered data is applied to sets of impedance rules for determining short circuit and open circuit lead related conditions. It is suggested that these rules and the testing process could be incorporated into the IPG to set a flag that identifies the lead defect and to emit a patient alert.
What the art has not yet shown is a self-testing system providing a lead status report that identifies particular lead related condition for each lead employed in the IMD. Optionally, such a monitor would cause a patient warning to be emitted and enable the IMD to alter its operating mode or to discontinue using a defective lead.